The invention relates to an impact additive of the core/shell type as well as to a composition containing a thermoplastic polymer, in particular a vinyl chloride homopolymer or a copolymer mostly containing vinyl chloride, an impact additive of the core/shell type and optionally other additives.
Some synthetic resins, in particular resins based on poly(vinyl chloride) or on a copolymer mostly containing vinyl chloride, are widely used in the building industry, in particular due to their low cost and to their good physical and/or chemical properties.
Nevertheless, they exhibit low impact strengths at ambient temperature or at low temperature or again also after ageing.
It has been proposed to overcome these defects by incorporating, in these thermoplastic resins, products known as impact additives which are generally polymers exhibiting a degree of elastomeric properties.
A description is given in U.S. Pat. No. 3,678,133 of an impact additive of the core/shell type composed of an elastomeric core and of a more rigid thermoplastic shell.
The elastomeric core is obtained by polymerization of a mixture of monomers comprising at least 50% by weight of an alkyl acrylate, the alkyl group of which has from 2 to 8 carbon atoms, and a minor proportion of a crosslinking agent. The preferred alkyl acrylate is n-butyl acrylate.
It is also mentioned that alkyl acrylates having longer chains exhibit the disadvantage of polymerizing with greater difficulty, 2-ethylhexyl acrylate being given as an example.
The rigid thermoplastic shell is obtained by polymerization of a mixture of monomers comprising 40% to 100% by weight of alkyl methacrylate in which the alkyl group contains 1 to 4 carbon atoms.
The impact additive in this patent is produced in such a way that the polymerization of the rigid thermoplastic shell takes place at the surface of the elastomeric phase, preferably as a separate layer which more or less completely covers the elastomeric core.
Although the impact additives thus obtained significantly improve the impact strength at ambient temperature of the resins containing them, there is a loss in the mechanical properties, in particular a loss in the impact strength, at low temperature, of the said resins.